The iptables rule is just saying that any incoming TCP segment with destination port 143 will be accepted and not e.g. DROPped when default chain policy is set to DROP or REJECTed when the segment is not matched by any rule and the last rule in the chain is REJECT.

If you want to see this port as opened and in listening state there has to be some application running and listening on that port. Apparently, there is not such app running so try to configure it first and then start it. The TCP port 143 is in general allocated for IMAP services (aka Dovecot, Courier-imap and so on).